An electric arc furnace is an electrically operated furnace used for melting metal and/or for cleaning slag. The operation of the furnace is based on an arc flame that burns either between separate electrodes, or between electrodes and the material to be melted. The furnace may be operated either by AC or DC current. Heat is created in the arc flame, and also in the material to be melted, in case the arc flame burns between the material and the electrodes. Power is conducted to vertical electrodes that are located symmetrically in a triangle with respect to the midpoint of the furnace. In the case of a DC smelting furnace there is one electrode in the middle of the furnace. The assembly depth of the electrodes in the furnace is continuously adjusted, because they are worn at the tips owing to the arc flame.
The lower part of the electrode column assembly comprises a contact shoe ring, a pressure ring and a heat shield. The contact shoe ring consists of a plurality of contact shoe elements arranged as a ring to be in contact with a steel mantle inside of which the electrode paste is sintered. Such an electrode is a so-called Söderberg electrode. The contact shoe elements conduct electric current to the electrode. A pressure ring is arranged on the outside of the contact shoe ring, so that the contact shoe ring is surrounded by said pressure ring. The pressure ring consists of a plurality of pressure blocks connected with each other as a ring, which pressure blocks being provided with hydraulic bellows by which the contact shoes in the contact shoe ring are pressed against the steel mantle of the electrode. A heat shield surrounding the electrode column assembly is arranged above the pressure ring in the axial direction of the electrode column assembly. Also the heat shield is comprised of a plurality of segments connected with each other to form an assembly of annular form.
Because of the extremely hot environment the contact shoe elements, pressure ring and heat shield must be cooled and in current electrode column assemblies cooling is arranged so that the contact shoe elements, pressure ring blocks and heat shield segments are provided with cooling channels for flowing cooling liquid. Cooling channels are made so that borings are drilled in the contact shoes, pressure ring blocks and heat shield segments. Borings are drilled so that they extend through said parts within which at least some of the borings are joining with each other. Therefore at least some of the borings must be closed with plugs. Plugging of the borings causes a risk for leaking of the cooling liquid. Leaks in the electrode column assemblies are very detrimental, especially when leaking plugs are situated in the hot lower end of the rings where they are not visible.